HIROSHIMA, Japan — Tens of thousands gathered in Hiroshima today to mark the 60th anniversary of the world’s first atomic bomb attack with a moment of silence and offerings of flowers and water.

More than 55,000 joined in the austere ceremony in Peace Memorial Park, a sprawling, tree-covered expanse that for one day each year becomes the spiritual epicenter of the global anti-nuclear movement.

A moment of silence was observed at 8:15 a.m. local time, the instant of the blast. Flowers and water — symbolizing the suffering of those who died in the atomic inferno — were offered at a simple, arch-shaped stone monument at the center of the park.

U.N. Secretary-General Kofi Annan, in a statement read in his name by a U.N. disarmament official, warned that, without concerted action, the world faced a “cascade of nuclear proliferation.”

Against a backdrop of raging disputes with Iran and North Korea over their nuclear aspirations, Mr. Annan said: “We are witnessing continued efforts to strengthen and modernize nuclear arsenals.

“We also face a real threat that nuclear weapons will spread,” he said.

The statement was read at a ceremony here by Nobuyasu Abe, the U.N. undersecretary for disarmament.

About 140,000 people were killed instantly or died within a few months after the Enola Gay dropped its deadly payload on the city, which then had a population of about 350,000.

Three days later, another U.S. bomber, Bock’s Car, dropped a plutonium bomb on the city of Nagasaki, killing about 80,000 people. Japan surrendered on Aug. 15, 1945, bringing World War II to a close.

The true toll on Hiroshima, however, is hard to gauge.

Including those initially listed as missing or who died afterward from a loosely defined set of bomb-related ailments, including cancers, Hiroshima officials now put the total number of the dead in this city alone at 237,062.

This year, about 5,000 names are being added to the list.

“For the people of Hiroshima, this is a day of prayer,” said Takaomi Tahara, who lost several relatives, including his grandfather, in the bombing.

To this day, he said, the remains of his dead relatives have not been found. “For us, there isn’t any closure.”

Along with being a time to remember those who died, Hiroshima’s anniversary has become the focus of the international peace movement.

In the biggest pre-anniversary event, about 8,000 people attended the annual World Conference Against Atomic and Hydrogen Bombs. The conference organizers, mainly leftist and labor groups, have collected more than 8.5 million signatures calling for a global nuclear ban.

On the eve of the anniversary, fundamentalist Christians held a prayer circle in Hiroshima, while members of the International Communist League handed out leaflets nearby.

Some people came on their own, offering a purely personal message.

“Our goal is to apologize to those who suffered and are still suffering the horrible, unspeakable atrocity of the atomic bomb,” said John Schuchardt of Ipswich, Mass., who came to Hiroshima with his wife. He said he was on a nine-day fast.